Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke are deposited on the catalyst. A high temperature regeneration operation within a regenerator zone combusts coke from the catalyst. Coke-containing catalyst, referred to herein as coked catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
Conventional regenerators typically include a vessel having a coked catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the gas exits the regenerator vessel.
There are several types of catalyst regenerators in use today. The conventional bubbling bed regenerator typically has just one chamber in which air is bubbled through a dense catalyst bed. Coked catalyst is added and regenerated catalyst is withdrawn from the same dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed.
Two types of regenerators have two chambers. Two-stage bubbling beds have two chambers. Coked catalyst is added to a dense bed in a first, upper chamber and is partially regenerated with air. The partially regenerated catalyst is transported to a dense bed in a second, lower chamber and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second chamber.
Complete catalyst regeneration can be performed in a dilute phase, fast-fluidized, combustion regenerator. Coked catalyst is added to a lower chamber and is transported upwardly by air under fast fluidized flow conditions while completely regenerating the catalyst. The regenerated catalyst is separated from the flue gas by a primary separator upon entering into an upper chamber in which regenerated catalyst and flue gas are disengaged from each other. Only a small proportion of air added to the regenerator vessel is added to the upper chamber. U.S. Pat. No. 4,197,189 and U.S. Pat. No. 4,336,160 teach a riser combustion zone in which fast fluidized flow conditions are maintained to effect complete combustion without the need for the additional combustion in the catalyst bed collected from the top of the riser.
After burn is a phenomenon that occurs when hot flue gas that has been separated from regenerated catalyst contains carbon monoxide that combusts to carbon dioxide. After burn can be a risk in an upper, disengaging chamber which contains hot flue gas that has been disengaged from catalyst, thereby providing a dilute catalyst phase. In this dilute phase of catalyst, insufficient catalyst is present to serve as a heat sink to absorb the heat of combustion thus subjecting surrounding equipment to potentially damaging higher temperatures and perhaps creating an atmosphere conducive to the generation of nitrous oxides.
Catalyst coolers have been used to cool regenerated catalyst and permit the regenerator and the reactor to operate under independent conditions. In catalyst coolers, hot regenerated catalyst is cooled by indirect heat exchange with water which vaporizes to steam. The steam is removed from the catalyst cooler for other uses; whereas, the cooled catalyst is returned to the regenerator. Air used to fluidize catalyst in the catalyst cooler can be vented to the regenerator.
Background to this invention is a catalyst cooler with a vent in communication with an upper chamber of a regenerator vessel. Air for fluidizing catalyst in the catalyst cooler and vented to the upper chamber can provide an oxidation agent for after burn.
Ways are sought to efficiently utilize air to fluidize hot catalyst in catalyst coolers for regenerators.